Four persons of the inventors proposed in the Japanese application No. Sho 55-13159 a semiconductor laser fabricated on a terraced substrate to oscillate with a stable transverse mode. The configuration of the preferred example of the proposed laser comprises layers and region as shown in FIG. 1,
a terraced substrate 1 of . . . n-GaAs having a terrace step thereon, PA1 a first clad layer 2 (a first epitaxial layer) of . . . n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 3 (a second epitaxial layer) of . . . non doped Ga.sub.1-y Al.sub.y As, PA1 a second clad layer 4 (a third epitaxial layer) of . . . p-Ga.sub.1-z Al.sub.z As, and PA1 a cap (current limiting) layer 5 (a fourth epitaxial layer) of . . . n-GaAs, and besides, PA1 a current injection region 6 . . . p-type Zn diffused region.
The first to fourth epitaxial layers 2 to 5 are formed on the terraced face of the substrate 1 by a known sequential liquid phase epitaxial growths. Therefore, the first clad layer 2 is formed to have triangular-section part at the step part, and active layer 3 has two bending at the upper end and the lower end of the triangular-section part defining an inclined region 31 therebetween. The inclined region has a larger thickness than other parts (upper horizontal part and lower horizontal part) and constitutes a strip shaped lasing region 31 wherein light oscillation is effectively confined. The triangular-section part of the first clad layer 2 is formed sufficiently thicker than the other parts of the first clad layer 2 and prevents leakage of the oscillated light therethrough, while in the other parts the oscillated light leaks out from the lasing region 31 therethrough, thereby suppressing undesirable oscillation in regions other than the lasing region 31. The second clad layer 4 is also formed thicker at the part on the inclined lasing region 31 than at the parts on the other parts of the active layer 3. The current limiting layer 5 (fourth epitaxial layer) is formed to grow in a manner to form its upper face substantially flat or horizontal at the part over the lasing region 31. The current injection region 6 is formed by diffusing Zn as p-type impurity in a strip shape pattern from the surface of the fourth epitaxial layer 5 in a manner that a corner of the diffused front goes into and remains in the second clad layer 4 at the part over the lasing region 31. Ohmic electrodes 7 and 8 are formed as the p-side electrode and n-side electrode, respectively, and the wafer which is manufactured to have arrays of a plural of laser unit thereon is cut into individual laser pieces by cleaving the wafer, then the unit laser is bonded on a heat sink stem 9 as shown by FIG. 2 upside down by bonding the p-side electrode 7 on the stem 9 by means of indium solder 10, to complete a laser device.
In the above-mentioned semiconductor laser proposed by four of the present inventors, though the performance is excellent, there is a problem of manufacturing that the diffusion front of the current injection region 6 should be controlled very accurately in order that the edge of the diffusion front reaches and remains in the second clad layer 4. In order to attain such accurate controlling, the current limiting layer 5 can not be made thick. Accordingly, at the peripheral part of the side of thicker part of the substrate 1, distance "l" from the surface of the p-side electrode 7 to the active layer 3, i.e., the total thickness of the layers 4, 5 and 7 has been fairly thin, thereby being liable to undesirable shortcircuiting of the active region 3 by the indium solder 10. That is, the pushed up parts of the molten solder 10 around the periphery of the semiconductor laser chip and irregular tip of the torn tip of the ohmic electrode 7 formed by cleaving of the wafer is likely to induce the shortcircuiting between the p-side ohmic electrode 7 and the active layer 3. This is because the short distance l, which is in actual device about only 2 .mu.m, is likely to induce a shortcircuit between the electrode 7 and the active layer 3 by the rise-up of the solder 10, when the semiconductor laser unit chip is put upside down on to the heat sink 9 and mounted thereon. Hitherto, such shortcircuitings have taken place in about 30 to 40% of the total manufactured semiconductor lasers.
Furthermore, since the current limiting layer 5 can not be made sufficiently thick in order to assure the accurate control of the diffusion to form the current injection region 6, the upper face of the p-side electrode 7 can not be made flat. Accordingly, the semiconductor chip is likely to be mounted undesirably tilted on the heat sink 9, and this also is likely to increase chances of the shortcircuitings.
Further, since the uppermost epitaxial layer (current limiting layer) 5 of the conventional device is GaAs layer, it is likely to occur that when a residue of GaAs solution remains on the surface of this uppermost layer, the surface evenness of the wafer is not good, resulting in poor contact between a photo-mask and the wafer surface in the subsequent photolithographic steps.
Thus, as a result of the above-mentioned shortcircuitings by the solder, the manufacturing yield of the terraced substrate type semiconductor lasers has not been sufficiently high.